The present invention relates generally to data receivers, and more particularly, to a system for use in a data receiver to optimize a detection point for a received data stream.
Optical networks are becoming widely used for distributing both high and low speed data over varying distances. Typically, an optical network is comprised of a number of network elements (NE) that are connected to each other in a variety of configurations so as to form a unified communication network. The communication network may extend over a small area, such as a company wide network, or may cover large distances, such as in regional or nationwide networks. Typically, the NE""s allow network clients to input data for transmission over the network and to receive data transmitted over the network from other locations. Thus, data may be added or dropped from the network at NE locations as the data flows from point to point throughout the network.
A network element may include one or more transceivers that convert optical signals from one wavelength to another, or from an optical to an electrical signal (or vice versa), so that for example, data received at a first wavelength and bit rate is sent out at different wavelength and bit rate. The transceiver may include both input and output tributaries that allow data to be received from and transmitted to network users that are local to the network element. For example, an input tributary allows a network user to input signals at an NE for transmission over the network, and an output tributary allows a network user to receive signals at an NE that have been received from the network.
With respect to the input tributary of a transceiver, a network client may provide data having an encoded clock signal for transmission over the network. For example, the data may be encoded using a non-return to zero (NRZ) encoding scheme, where a xe2x80x9c0xe2x80x9d is the absence of a pulse, and a xe2x80x9c1xe2x80x9d is the presence of a pulse. Furthermore, different network clients may input data having different data rates. As a result, one problem that exists is that network operators are required set up the input tributaries to receive each client""s data.
A tributary""s input receiver may include a clock and data recovery (CDR) circuit that recovers a clock signal to associate with the received data stream. The clock signal is used by downstream processing logic to detect the data in the data stream. For example, the data stream may be processed by error correction logic prior to transmission on the network. The error correction logic uses the clock signal to detect the data and to encode error correction information with the data that will be used by a destination processor to correct any data errors that occur during transmission over the data network.
Unfortunately, one or more problems may occur if the recovered clock signal is not precisely aligned to the optimal detection point of the data stream. For example, the clock signal edges may occur slightly earlier or later than the optimal point in each data bit, resulting in an increase in the bit error rate (BER) associated with the data stream. Furthermore, setting a basic amplitude threshold for the data, for instance 50%, may not provide the best detection result. The mis-alignment of clock and amplitude threshold values results in significant detection problems, especially when the input power level of the data stream is low, so that even small mis-alignments result in many detection errors. As the detection errors increase, even the effectiveness of error correction processing is limited.
Therefore, it would be desirable to have a way to fine tune the amplitude and phase detection points of a data stream to improve data detection, especially at low input power, and thereby reduce data errors and increase the effectiveness downstream processing.
The present invention includes a system to fine-tune the amplitude and phase detection points of an incoming data stream. For example, in a receiver system, an incoming data stream may be input to a clock recovery circuit that generates one or more clock signals from the received data stream. However, the generated clocks may not be properly aligned to the data to provide the best data detection. Furthermore, the selected amplitude threshold value may not be optimal, and so, additional errors may occur. Thus, the system included in the present invention operates to fine-tune the detection point, in both amplitude and phase, so that the data can be optimally detected.
One or more embodiments included in the present invention operate to fine-tune the detection threshold by processing an xe2x80x9ceye patternxe2x80x9d associated with the data stream. The eye pattern is a representation of the data stream and includes both amplitude and phase characteristics of the data. For example, a horizontal axis represents the data""s phase characteristics and the vertical axis represents the data""s amplitude characteristics.
In one embodiment, the system processes an eye pattern associated with the data to determine an optimal phase opening value and an optimal amplitude opening value. The optimal phase and amplitude opening values represent the best phase and amplitude detection regions in the eye pattern. The two opening values are used to determine an optimal threshold point that represents the best threshold point with which to detect the data. In one embodiment of the invention, the optimal threshold point is determined at periodic intervals so that the threshold point can be adjusted if the data""s eye pattern changes over time. In another embodiment, the optimal detection threshold is determined and then stored in a memory, so that it may be retrieved for use at a later time.
Embodiments included in the present invention may be used in a variety of applications, including both electrical and optical transmission systems. For example, the system may be used in a network transceiver so that a clock signal recovered from a data stream may be fined tuned to the optimal detection threshold. The resulting transceiver provides excellent data detection, even at low power levels, and improves the operation of downstream processing systems, for example, error correction logic.
In one embodiment of the invention, a system for determining an optimal detection point associated with a data stream is provided. The data stream has an associated eye pattern that contains a data value for each of a plurality of detection points defined by a plurality of amplitude rows and a plurality of phase columns. The system includes an amplitude processor operable to determine an amplitude opening value for each of the phase columns, wherein for each of the phase columns the amplitude opening value indicates a total number of the plurality of amplitude rows that contain a data value that is below a selected threshold value. The amplitude processor also operates to determine a selected phase column based on the amplitude opening values.
The system also includes a phase processor operable to determine a phase opening value for each of the amplitude rows, wherein for each of the amplitude rows the phase opening value indicates a total number of the plurality of phase columns that contain a data value that is below the selected threshold value. The phase processor also operates to determine a selected amplitude row based on the phase opening values.
The system also includes control logic coupled to the amplitude processor and the phase processor. The control logic is operable to determine the optimal detection point based on the selected phase column and the selected amplitude row.
In another embodiment of the invention, a method is provided for optimizing a detection point associated with a data stream. The data stream has an associated eye pattern that contains a data value for each of a plurality of threshold points defined by a plurality of amplitude rows and a plurality of phase columns. The method includes a step of determining an amplitude opening value for each of the phase columns, wherein for each of the phase columns the amplitude opening value indicates a total number of the amplitude rows that contain a data value that is below a selected threshold value.
The method also includes a step of determining a phase opening value for each of the amplitude rows, wherein for each of the amplitude rows the phase opening value indicates a total number of the plurality of phase columns that contain a data value that is below the selected threshold value.
The method also includes a step of determining a selected amplitude row from the phase opening values and a selected phase column from the amplitude opening values, and a step of determining the optimal detection point from the selected phase column and the selected amplitude row.